1. Field of the Invention
The present invention relates to a method for producing a feed material for molten metal production, and a method for producing a molten metal. Particularly, the present invention relates to a method for producing reduced iron (molten iron) from an iron oxide-containing material such as an iron ore, steel mill waste, or the like by using biomass as a reducing agent.
2. Description of the Related Art
An example of a technique for utilizing biomass is a technique for generating electric power by gasifying biomass such as organic waste, organic sludge, livestock waste, municipal solid waste, or the like, waste such as waste plastics or the like, or low-quality fossil fuel such as coal, heavy oil, or the like. In this technique, a methane gas obtained from biomass is burned and used as a heat source for generating electric power. However, this technique comprises gasification and combustion steps to complicate the process and increase the cost.
Another example comprises carbonizing biomass to obtain fuel, a water purifying material, or vegetable coal used as a soil conditioner. However, there are only few applications of the resultant carbide.
On the other hand, a process comprising incorporating a carbonaceous material into an iron oxide-containing material such as an iron ore, steel mill waste, or the like, and heating the iron oxide-containing material in a rotary hearth furnace to obtain reduced iron is also known. This process uses coal or coke as the carbonaceous material, and thus the carbonaceous material is expensive.
However, an inexpensive biomass carbide (carbonized biomass) is thought to be used as the carbonaceous material. If the biomass carbide can be used as the carbonaceous material, the utilization of biomass can be further advanced.
In the technical field of reduced iron production, a method using biomass in some form comprises treating composite pellets composed of iron oxide and a solid carbonaceous reducing agent in a reducing furnace to form a directly reduced iron product. In this method, a biomass substance such as wood waste or the like can be used as the solid carbonaceous reducing agent.
However, in this method, in treating the composite pellets composed of biomass and iron oxide in the reducing furnace, air or oxygen is introduced into the reducing furnace, and in this state, the biomass in the composite pellets is thus simply burned fail to secure a sufficient amount of biomass carbide as the reducing agent. For example, the wood waste used as the biomass contains 30 to 50% by mass (% by weight) of organic carbon contents and 3 to 15% by mass of inorganic carbon contents. However, under the above-described condition, the organic carbon contents are mostly burned, and many of the inorganic carbon contents are also burned.
Also, the composite pellets are produced from biomass and iron oxide without a treatment of the biomass. However, this method has a great difficulty in producing the composite pellets, and even when the composite pellets obtained by this method are treated in the reducing furnace in which air or oxygen is not introduced, the resultant biomass carbide does not effectively function as the reducing agent.
Namely, biomass is generally several mm or more in size, for example, wood waste is 10 to 100 mm in length. Therefore, when biomass used as the reducing agent is agglomerated with iron oxide to form the composite pellets, the biomass must be ground before agglomeration, mixed with iron oxide, and then agglomerated. However, wood biomass such as wood waste or the like is fibrous, and it is thus very difficult to pulverize the wood waste to a powder (it is difficult to split a fiber, and thus at least plural types of grinding steps are required). Similarly, construction and demolition waste, thinned wood, and sawmill waste cannot be easily pulverized, and organic waste and municipal solid waste also cannot be easily pulverized. Therefore, it is difficult to pulverize biomass to a powder, and thus it is very difficult to agglomerate biomass with iron oxide.
Even if biomass can be agglomerated with iron oxide without being pulverized, and even if biomass can be treated to produce a biomass carbide in a reducing furnace in which air or oxygen is not introduced, the biomass carbide has little opportunity of contact with iron oxide (small contact surface area) to fail to effectively reduce iron oxide unless the biomass is pulverized to a powder and then agglomerated with iron oxide.
Furthermore, biomass (slurry sludge) with a high water content cannot be easily agglomerated before drying.
On the other hand, as a conceivable measure for avoiding difficulties in agglomeration with iron oxide, biomass is singly carbonized without coexisting with iron oxide. However, biomass such as wood chips or the like generally has a low specific gravity (light) and low thermal conductivity, and thus carbonization of single biomass has low efficiency. In regard to the specific gravity, for example, the bulk specific gravity of wood chips is 0.155 g/cm3 which is considerably lower than the bulk specific gravity of 0.95 g/cm3 of electric furnace dust (an iron-oxide containing material).
In such a process for carbonizing biomass singly, liquid biomass with a high water content also has a problem of unstable treatment because the biomass does not stably stay in a furnace, for example, a rotary kiln, during the treatment. Furthermore, some kind of biomass is easily agglomerated or adhered to the furnace wall to cause a problem about treatment stability and maintenance. Furthermore, the process has a defect that a carbonization gas (reducing gas) produced in carbonization of biomass is not at all utilized for reducing iron oxide.